The forum normally defines spoilers as material from a book currently in its first year (generally any book only available in hardcover). So right now the only real spoiler material is the Cold Days sample chapters. On the other hand not everyone (especially in the RPG board) has read all the books, and since the RPG stops around Small Favor that seems to be around where you should start being careful of spoilers.
That said The Fomor are supposed to remind you of Dagon. In HP Lovecraft's works Dagon is an Old One and in older mythologies it is the "fish god." In the Lovecraftian mythos Dagon is the "leader" of the Deep Ones who are basically fish people. The movie Dagon is based on HP Lovecraft's "The Shadow over Innsmouth" which is one of the main stories he wrote about Dagon. Jim Butcher has hinted that the most powerful Outsiders are his worlds interpretation of HP Lovecraft's Old Ones, so it makes sense that there would be a Dogon analog in the books.